Light olefins, defined herein as ethylene and propylene, serve as feeds for the production of numerous important chemicals and polymers. Light olefins traditionally are produced by cracking petroleum feeds. The escalating cost of petroleum feeds has led to the development of new technology for producing light olefins, such as oxygenate-to-olefin (“OTO”) technology.
In an OTO reaction system, a feedstock containing an oxygenate is vaporized and introduced into a reactor. Exemplary oxygenates include alcohols such as methanol and ethanol, dimethyl ether, methyl ethyl ether, methyl formate, and dimethyl carbonate. In a methanol to olefin (MTO) reaction system, the oxygenate-containing feedstock includes methanol. In the reactor, the methanol contacts a catalyst under conditions effective to create desirable light olefins. Typically, molecular sieve catalysts have been used to convert oxygenate compounds to olefins. Silicoaluminophosphate (SAPO) molecular sieve catalysts are particularly desirable in such conversion processes because they are highly selective in the formation of ethylene and propylene.
MTO reactor systems can form undesirable byproducts through side reactions. For example, the metals in conventional reactor walls may act as catalysts in one or more side reactions. If the methanol contacts the metal reactor wall at sufficient temperature and pressure, the methanol may be converted to undesirable methane and/or other byproducts. Byproduct formation in an MTO reactor is undesirable for several reasons. First, increased investment is required to separate and recover the byproducts from the desired light olefins. Additionally, consuming feed in byproduct reactions results in less feed being available for the desired light olefins, thereby reducing light olefin yield. While the relative concentration of metal-catalyzed side reaction byproducts is generally quite low, the total amount of byproducts produced on an industrial scale can be very large. Thus, it is desirable to decrease or the amount of byproducts produced by an MTO reactor system. One way to do this is by deactivating or passivating reactor surfaces.
Sulfur-containing chemicals have proven effective for deactivating or passivating the metal surface of a reactor thereby reducing the formation of undesirable byproducts in the reactor. For example, Japanese Laid Open Patent Application JP 01090136 to Yoshinari, et al. is directed to a method for preventing decomposition of methanol or dimethyl ether and coking by sulfiding the metal surface of a reactor. More particularly, the method includes reacting methanol and/or dimethyl ether in the presence of a catalyst at above 450° C. in a tubular reactor made of Iron and/or Nickel or stainless steel. The inside wall of the reactor is sulfided with a compound such as carbon disulfide, hydrogen disulfide or dimethyl sulfide. Additionally, a sulphur compound may be added to the feed.
Byproduct production can also be curtailed by reducing the amount of feed decomposition in the MTO reaction system's feed vaporization and injection region. For example, U.S. Pat. No. 7,034,196, discloses a method for reducing methanol decomposition by regulating the temperature of the feed injectors. Other references, such as U.S. Pat. Nos. 7,338,645 and 6,737,556, disclose coating a portion of the feed injectors with materials that are not catalytically active for methanol decomposition.
While these methods have proven effective, further reductions in the amount of methanol feed decomposition and/or the conversion to undesirable byproducts are desired.